The invention relates to resistors for electronic and electrical circuits and particularly to a four terminal safety resistor. The invention has particular application to certain fail safe circuits. One type of fail safe circuit is, for example, found in railway signalling circuits. In the event of failure such circuits must fail in a manner that will cause the train or other control system to default to a more restrictive operating mode. For example, in the case of a train control system this is necessary to prevent the possibility of an accident.
For the design of various equipment it is desirable to have a compact 4-terminal safety resistor, which can be mounted on printed circuit boards. The prior art includes conventional two terminal resistors of the "established reliability" type that have passed government lead integrity tests. These resistors are mounted in such a manner that two independent connections are made to each of the two leads. Such devices have been installed in emitter voltage divider circuits and other circuits which demand fail-safe performance characteristics. Such arrangements required a substantial area on a printed circuit board, were expensive to manufacture, and required special handling.
The prior art also includes several four terminal electrical components. These include a four terminal varistor described in U.S. Pat. No. 3,959,763. This patent, having the same assignee as the present application, describes a structure which, in one form, utilizes conventional semiconductor material which have contact metalization on opposite sides of the material. Two discreet leads extend from each of the contact metalization areas.
U.S. Pat. No. Re. 29,126 based on U.S. Pat. No. 3,792,323 describes a wound capacitor that has four discrete leads. U.S. Pat. No. 4,614,906 discloses a means for connecting a plurality of loads in series across a voltage source without encountering reliability problems due to failure of any portion of the series circuit. U.S. Pat. No. 3,614,466 discloses a method for using a photopositive resistive element in an amplifier circuit such that the circuit will not produce an output during a component failure in a feedback loop. U.S. Pat. No. 4,528,610 discloses a means for isolating a section of a circuit from a supply in the event of a short circuit in the section.
These prior art devices and circuits do not satisfy the requirements for various circuits such as the emitter terminal voltage divider of a safety critical circuit.
It is an object of the invention to provide a compact 4-terminal resistor.
It is an object of the invention to provide a 4-terminal resistor that, in the event of a rare failure, has a failure mode that causes a default of the control circuit of which it is a part to a more restrictive manner of operation.
It is an object of the invention to provide a 4-terminal resistor which is inexpensive to manufacture.
Still another object of the invention is to provide a 4-terminal resistor which is particularly adapted for mounting on a printed circuit board.